(1) Field of the Invention
The present invention relates to a combined in-plane shear loading and multi-axial tension or compression testing apparatus in which the apparatus is capable of determining the mechanical properties of metals, plastics, woods, fabrics, elastomers and other materials.
(2) Description of the Prior Art
Plain-woven fabrics are widely utilized as structural materials in air-inflated structures and rapidly deployable structures such as temporary shelters, tents, temporary bridges and space structures. Unlike metallic structures, these structures are primarily designed to be lightweight, self-erecting and deployable to volume-storage ratios that may be 1000-to-1. Air-inflated structures utilize pressurized fabric tubes and pressure-stabilized beams (known as air beams) as load-carrying members.
Although, the structures are well-known in the art, the technology for the structures has not been refined such that reliable structures can be analytically designed. Specifically, this analysis has gained in importance due to advancements in the material of the structural fiber and the weaving/braiding of the structural fiber, both of which have improved the load carrying capacity of the structures. Accordingly, there is a recognized need to model the mechanical properties of woven fabrics.
Presently, modeling the mechanical properties of woven fabrics results in complex responses because of the complex microstructures of the composite materials used. Unlike traditional composite materials, plain-woven fabrics used in inflated structures exhibit high non-linearity with a dependence on internal pressure and contact interactions within the woven fabric.
Accordingly, there is a need for a testing apparatus which measures the elastic and shear moduli of air beams as a function of inflation pressure. To measure the elastic modulus of the fabric, a multi-axial loading has been shown to be preferable and to measure the shear moduli of the fabric; an in-plane shear loading has been shown to be preferable. As such, there is a need for a testing apparatus capable of combining in-plane shear and multi-axial loading. For non-orthogonal composite or fabric materials, such as braids or knits, there is a further need for a testing apparatus capable of loading the specimen in varying non-orthogonal positions.
While biaxial testing apparatuses with compression and tension loading or in-plane shear testing apparatuses exist in the prior art, there are no apparatuses that exist with a combined feature of in-plane shear and compression/testing capabilities. Also, a testing apparatus does not exist that is capable of applying non-orthogonal multi-axial loading.
Additionally, testing apparatuses of the prior art employ two or more separate actuators in complex test fixtures or pressurization techniques for applying a biaxial load to a test specimen. An apparent disadvantage is the need for two or more loading devices and the associated high cost of equipment.
In regard to specific references, Lynch et al. (U.S. Pat. No. 3,776,028) describes an apparatus requiring three independent loading mechanisms. Holt (U.S. Pat. No. 4,192,194) describes an apparatus for biaxial loading of a specimen by pressurizing the inside surface of a cylinder. A restrictive disadvantage of the apparatus is the requirement of the cylindrical shape of the specimen and a high cost associated with pressurization of the cylinder. Additionally, the disadvantages include restriction to orthogonal loads about the axial, hoop and radial directions and an apparatus that is not capable of applying an in-plane shear stress to the specimen.
Mathiak et al. (U.S. Pat. No. 5,144,844) describes a cruciform planar specimen for biaxial material testing which has the disadvantage of being limited to use in two loading directions. Ward et al. (U.S. Pat. No. 5,279,166) describes an apparatus for self-alignment of a biaxial loading device. The apparatus requires that the two axial loading directions be orthogonal with a maximum of two loading directions. The apparatus also has no capability for applying an in-plane shear load to the specimen.
Tucchio (U.S. Pat. No. 5,448,918) describes an apparatus with an X-shape that is only used for compression load. Clay (U.S. Pat. No. 5,905,205) describes an in-plane biaxial test apparatus comprising linkages to transfer the load to an orthogonal direction of loading. A disadvantage of this apparatus is that it is not capable of applying in-plane shear to the test specimen. Another disadvantage of this apparatus is that the biaxial loading is limited to an orthogonal configuration.
As noted above, none of the references are capable of combining the in-plane and compression/tension loading of a specimen while only using one loading system. As such, there exists a need for an apparatus capable of applying a combined in-plane shear and tension/compression load to a specimen. Such an apparatus would be cost-effective due to reduced space and a reduced amount of equipment normally needed for material testing.